Add an action to "Discard unpublished edits"

[vickychijwani]

This would be a pretty common use-case I believe. So common, indeed, that it might be worthwhile to show a tip in a Snackbar, when a published post is auto-saved for the first time, saying "We've saved those edits on your device. If you don't want them, use the "Discard unpublished edits" action." with a "Show me" button on the Snackbar that opens the overflow menu in the post editor automatically.

This could be implemented by simply overwriting the local ("dirty") copy of the post with the canonical copy from the server, but then it won't work offline.